This invention relates to lighting systems for discharge lamps, and pertains more particularly to a lighting system having an inverter and associated means for control of the inverter output frequency for harmlessly and quickly lighting up a discharge lamp as typified by a fluorescent lamp. Still more particularly, the invention concerns, in such a lamp lighting system, how to protect the switch or switches of the inverter against destruction due to overcurrent.
It has been known and practiced conventionally to incorporate an inverter in discharge lamp lighting systems for higher lighting efficiency, among other purposes, as disclosed for example in Japanese Patent No. 2627740. Such known lighting systems having an inverter are alike in including a resonant circuit of an inductor and a capacitor connected in series between the pair of output terminals of the inverter, with the discharge lamp connected in parallel with the capacitor. The discharge lamp has its pair of filamentary electrodes connected in series with the capacitor in order to be preheated before being lit up.
The magnitude of the current flowing through the LC resonant circuit is frequency dependent, growing to a maximum at a resonance frequency and diminishing in both increasing and decreasing directions from that frequency, because both inductor and capacitor of the resonant circuit inherently possess resistive components. Consequently, the voltage across the capacitor also maximizes at the resonance frequency and diminishes in both directions from that frequency.
As is well known, an electron radiating substance is coated on the filamentary electrodes of the discharge lamp. In a lighting system including an inverter, the lamp electrodes are preheated as aforesaid, instead of being suddenly subjected to a voltage high enough to initiate an electric discharge therebetween, in order to prevent the electron radiating substance from vaporizing or scattering away from the filaments. The preheating of the lamp electrodes are accomplished by maintaining the voltage across the capacitor at a constant value less than the voltages applied during the subsequent lightup period. The lamp is then lit up by decrementing the inverter output frequency and thereby incrementing the voltage across the capacitor until the lamp starts glowing with the commencement of a discharge between the lamp electrodes.
In discharge lamp lighting systems of the above known constructions, there have been a problem left unsolved in connection with the switch, or the pair of switches, of the inverter. An abnormally high current would flow through the inverter switch or switches if the current of the LC resonant circuit were in phase advance with respect to the inverter output voltage. The inverter switch or switches would be ruined with the repeated flow of such overcurrent.
It is known, however, that the LC resonant circuit operates as inductive reactance at frequencies above the resonance frequency, and as capacitive reactance at frequencies below the resonance frequency. The current flowing through the resonant circuit is in phase delay when it is operating as inductive reactance, and in phase advance when it is operating as capacitive reactance. The inverter is therefore driven so as to provide output frequencies above the resonance frequency of the resonant circuit in order to preclude the danger of destruction of the inverter switch or switches.
As has been mentioned, the lamp is lit up by decrementing the inverter output frequency from a predetermined value f.sub.1. in FIG. 6 of the drawings attached hereto) above the resonance frequency (f.sub.0) until the lamp starts glowing (as at f.sub.2). The voltage required for holding the lamp glowing can be less than its discharge start voltage, so that the inverter output frequency is further reduced after the lamp has been lit up, and fixed at a value (f.sub.3) that is less than the resonance frequency (f.sub.0) of the LC resonant circuit. However, on being lit up, the discharge lamp becomes electrically connected in parallel with the resonant capacitor. The resonant frequency (f.sub.4) of the resulting resonant circuit, inclusive of the glowing discharge lamp, is less than that (f.sub.0) of the LC resonant circuit exclusive of the lamp and, indeed, the normal output frequency (f.sub.3) of the inverter. Thus the inverter output frequency (f.sub.3) remains higher than the resonant frequency (f.sub.4) when the lamp is glowing, too, holding the current of the resonant circuit in phase delay and so saving the inverter switch or switches from overcurrent destruction.
The statement of the preceding paragraph holds true, however, only in the case where the discharge lamp is in good working state. Near the end of its service life in particular, the lamp may accidentally go off while being energized with the inverter output frequency at the normal value (f.sub.3). Thereupon this normal frequency will become less than the resonant frequency (f.sub.0) which is then determined by the LC resonant circuit exclusive of the discharge lamp. Conventionally, the resulting phase advance of the resonant circuit current have caused the flow of overcurrent to the inverter switch or switches, destroying them in the worst case.
The same accident has occurred with totally malfunctioning or used-up discharge lamps that remain unlit when the inverter output frequency is reduced as above for lighting them up.
An obvious remedy to this inconvenience might seem to hold the inverter output frequency above the resonant frequency (f.sub.0) of the resonant circuit exclusive of the discharge lamp when the lamp is unlit, and hence to prevent current flow through the resonant circuit in phase advance. This solution is unsatisfactory, bringing about other inconveniences, because of the narrowing of the inverter output frequency range, or of the voltage range of the resonant capacitor, that could be utilized for lighting up the lamp.